将lambda直接分配给继承的抽象方法[Kotlin]

Question

I am subclassing a BroadcastReceiver and would like to be able to specify a lambda as an input parameter and assign it directly to be used as the implementation for "onReceive". This is how my code looks like:

class Receiver(val callback: (Context?, Intent?) -> Unit): BroadcastReceiver(){
    override fun onReceive(context: Context?, intent: Intent?) = callback

    fun itWorks(context: Context?, intent: Intent?) = callback
}

I would like to be able to instantiate it like:

val myReceiver = Receiver({context: Context?, intent: Intent?-> {
    println("Intent received: $intent")
}})

Or

val myReceiver2 = Receiver(::implementationHere)

However I get the following error:

Error:(2, 18) Return type of 'onReceive' is not a subtype of the return type of the overridden member 'public abstract fun onReceive(p0: android.content.Context!, p1: android.content.Intent!): kotlin.Unit defined in android.content.BroadcastReceiver'

Interestingly, I am not getting an error when assigning this lambda to the "itWorks" method. Could you please tell me what's the difference between these two methods? Why does it work with one but not the other? As far as I can tell the signatures are the same in both methods.

(I'm coming from a Java8 background, where (as far as lambdas go) all you care about is to have the same method signature)

Of course this works, but I would prefer the other way:

class Receiver2(val callback: (Context?, Intent?) -> Unit): BroadcastReceiver(){
    override fun onReceive(context: Context?, intent: Intent?) {
        callback(context, intent)
    }
}

Answer 1

You need to invoke your lambda callback with parameters:

    override fun onReceive(context: Context?, intent: Intent?) = callback(context, intent) // returns Unit

This works only because the method's return type becomes (Context?, Intent?) -> Unit (with the method parameters being unused):

    fun itWorks(context: Context?, intent: Intent?) = callback // returns (Context?, Intent?) → Unit